Stabilized gas-in-liquid emulsions are useful in a variety of fields, such as food technology, marine biology, hydraulic and ocean engineering and echocardiography, to name a few. Gas microbubbles are particularly useful as contrast agents for ultrasonic diagnostics of fluid-filled human and animal body cavities and organs. In ultrasonic diagnostics, contrast agents provide better contrast resolution between normal and diseased cavities; outline vessels; characterize tissue; enhance Doppler signals in blood flow measurements; and are useful in dynamic studies, for example, to measure the rate of uptake and/or clearance of an agent in a specific location of the body. Contrast agents are particularly useful in echocardiography since injected air microbubbles travel with intracardiac velocities similar to red blood cells. Such microbubbles permit monitoring of blood flow which shows changes both in tumor neovascularization and in normal vascularization patterns of organs neighboring abdominal masses. This can provide earlier diagnosis of these abdominal masses.
There are three potential theoretical mechanisms for enhancing an ultrasound image, namely, increasing sonic backscatter, increasing the rate of attenuation of sound energy and altering the speed of transmission of ultrasound waves. Conventional ultrasound devices rely on generation of an image from backscattered ultrasound radiation. The term "echogenicity" refers to the degree of enhancement of backscatter. The echogenicity of an ultrasound contrast agent depends on experimental conditions and differences in physical properties of the scatterer and the suspending medium. Sonic backscatter may be increased by including free gas bubbles in a suspending medium. However, such free gas bubbles are short-lived and are quickly and completely removed by the lungs.
Typical prior art contrast mediums include encapsulated gas bubbles that exhibit better stability than free gas bubbles. However, such prior art encapsulated gas bubbles generally have a mean diameter greater than 10 .mu.m and can become entrapped in the capillary bed of the lung. Conventional microbubbles have been encapsulated in gelatin and albumen, for example. ALBUNEX.RTM., which has been developed by Molecular Biosystems, Inc. of San Diego, Calif., is a suspension of stable microencapsulated air bubbles of a size ranging from 0.5 to 10 .mu.m. The air bubbles are produced by sonication of a 5% human serum albumin solution. The gas bubbles are encapsulated in a coagulated protein shell. The microcapsules tend to be rather fragile in nature, and encounter problems in the high pressure environment of the chambers of the heart.
One agent which has been used to generate microbubbles is a sugar molecular matrix (SHU-454 and SHU-508), which is supplied as a crystalline solid by Schering AG (West Germany). Microbubbles are liberated from the matrix by addition of a sterile buffer solution just prior to administration, and range in size from 1 to 5 .mu.m.
U.S. Pat. No. 4,684,479 discloses surfactant mixtures for the production of stable gas-in-liquid emulsions comprising: (a) a glycerol monoester of saturated carboxylic acids containing from about 10 to about 18 carbon atoms or aliphatic alcohols containing from about 10 to about 18 carbon atoms; (b) a sterol-aromatic acid ester; and (c) a sterol, terpene, bile acid or alkali metal salt of a bile acid. Optionally, the mixture may also include various sterol esters, esters of sugar acids and aliphatic alcohols, sugar acids, saponins, sapogenins and glycerols.
U.S. Pat. No. 4,466,442 discloses a solution for the production of gas microbubbles which contains a solution of at least one tenside and at least one viscosity-raising compound. Examples of suitable non-ionic tensides include polyoxyethylene fatty acid esters, and polyoxyethylated sorbitan fatty acid esters. Examples of viscosity-raising compounds include mono- or polysaccharides, dextrans, cyclodextrins, hydroxyethyl amylose, polyols, proteins, proteinaceous materials, amino acids and blood surrogates.
Desirable contrast agents improve resolution for imaging of cardiac, solid organ and vascular anatomic conduits (including agent localization, for example due to macrophage activity); solid organ perfusion; and Doppler signals of blood velocity and flow direction during real time imaging. For an ultrasound contrast agent to be effective, it must be stable, biocompatible, and must provide improved acoustic echoes from tissue with relatively low toxicity.
It is desirable to have an ultrasound contrast agent comprising microbubbles having a mean average diameter less than 10 .mu.m which are stable not only in storage but are also stable for at least one pass inside a human or animal subject and which exhibit a high degree of echogenicity under all modes of medical ultrasound imaging.